This invention relates to optical amplifiers and is concerned more particularly, but not exclusively, with erbium doped fibre amplifiers (EDFAs).
Optical amplifiers designed to amplify multichannel optical signals ideally maintain gain tilt substantially constant as the gain level, that is the set point, is varied. This should be done by careful control of gain and loss within the amplifier so that, at each gain set point, a substantially constant gain tilt is applied across the range of wavelengths to be amplified. Preferably the gain tilt is compensated so that the gain is substantially constant across the full range of wavelengths to be amplified. In the case of an erbium doped fibre amplifier (EDFA) the gain is controlled by the drive current applied to the or each optical pump laser for supplying pump light to the erbium doped fibre (EDF) loop, and a compensating loss may be provided by a variable optical attenuator (VOA) so as to control the gain such that a substantially constant gain is applied across the full range of wavelengths for a particular gain set point. A pump laser having a linear drive response and control can be provided by analogue or digital techniques. However, VOAs have variable characteristics with the result that it can be difficult to ensure that the gain tilt applied to the multichannel signal remains substantially constant at different levels of amplification.
In this specification the term “gain tilt” encompasses within its scope both the case where the “gain tilt” is zero, corresponding to all the channels being amplified by the same amount, and the case where the “gain tilt” is non-zero.
FIG. 1 is a graph illustrating a change in the gain of such an amplifier with respect to time, the arrows showing diagrammatically the ideal case in which, at each gain level, the different channels are amplified by the same amount (zero gain tilt) whether the amplification is at its initial low level, an intermediate level or its final high level.
However, FIG. 2 shows a typical VOA drive characteristic from which it will be appreciated that not only does the VOA loss vary non-linearly with the applied voltage, but also a different voltage change is required for the same loss change, for example 3 dB, depending upon the magnitude of the loss as shown by A (low VOA loss, high amplifier gain) and B (high VOA loss, low amplifier gain) in FIG. 2, where clearly the voltage change at A is much greater than at B even though they both correspond to the same loss change.
Typically a digital control circuit employing a simple proportional-integral (PI) loop is used to control the VOA loss to render the gain for the different wavelength channels substantially the same across all channels. However such a circuit is unable to control the loss in the required manner at both points A and B. Instead the provision of a high gain factor in the PI loop will mean that there is an overshoot of the required VOA loss at high voltage and instability at low VOA loss. A low gain factor in the PI loop will mean that the VOA loss is not achieved in the required time frame and the loss lags the amplifier gain at low VOA drive.
Since the gain of the amplifier and the VOA attenuation must be changed simultaneously in order to control the tilt, the VOA attenuation cannot be changed quickly enough in the case of there being a low gain factor in the PI loop, and accordingly the VOA attenuation falls behind the value (or target) required. This results in the output spectrum being tilted until such time as the VOA attenuation catches up the amplifier gain. This lag is shown diagrammatically in FIG. 3 for a VOA control arrangement utilising a low gain factor in the PI loop showing the variation 1 in the attenuation with respect to time, being a stepwise variation with a cycle time of 16 ms and a target step of 0.1 dB, adjacent to the gain variation 3 with respect to time of the amplifier. In this case the 16 ms cycle of the VOA step voltage results in the VOA target being met at high VOA losses, as shown at 2, whereas, as the VOA step voltage increases, the target is increasingly not met, resulting in a lag between the target step time and the VOA arrival time, as shown at 4. This time lag at low VOA losses results in the gain tilt not being maintained so that some channels are subjected to greater gain than is necessary. This is shown in the graph of FIG. 4 corresponding to that of FIG. 1 but showing, by means of the different sized arrows, that, during a set point change, some of the channels are subjected to a greater gain than other channels at an intermediate gain level (non-zero gain tilt), whereas the different channels are amplified by the same amount (zero gain tilt) when the amplification is at its initial low level or its final high level, so that the gain tilt varies as the gain set point is changed. Measurement of the channel gain at this intermediate gain level clearly shows an overshoot in the gain of the blue-end channels.
FIG. 5 shows the change in the log optical power with respect to time of the 1530 nm and 1556 nm channels of such an amplifier during such a gain set point change from a low gain to a high gain. It will be seen in this figure that, during the rising section of the plot, the ramp rate is fairly similar for the two channels for the majority of the time, but that an overshoot can be observed in the 1530 nm channel due to the lag in the VOA loss reaching its target. This can be seen more clearly in the enlarged view of FIG. 6 showing that the tilt control begins to fail at high gains when the voltage change for the same VOA loss is less. In this case both channels reach the top of the ramp at the same time indicating synchronised digital signal processing (DSP) and VOA.
FIG. 7 shows the corresponding case for a gain set point change from a high gain to a low gain, with the initial part of the transition corresponding to a relatively low VOA loss resulting in dispersion of the channels as a result of the VOA loss not reaching its correct value. At higher VOA loss values, the VOA loss is able to catch up to the required set point and accordingly the dispersion between the channels is less.